1. Field of the Invention
The present invention relates to high temperature, erosion resistant coatings, and more particularly relates to the use of such coatings as abradable seals and thermal barrier coatings.
2. Background Information
Most components of combustion turbines are operated at very high temperatures and often require the use of specialized coatings/inserts to protect underlying supporting materials. These specialized coating/inserts include thermal barrier coatings (TBCs), disposed opposite the turbine blade tips, as taught in U.S. Pat. No. 5,180,285 (Lau).
Conventional TBCs typically comprise a thin layer of zirconia. In many applications, the coatings must be erosion resistant and must also be abradable. For example, turbine ring seal segments, which fit with tight tolerances against the tips of turbine blades, must withstand erosion and must also preferentially wear or abrade in order to reduce damage to the turbine blades, and form a tight seal with the turbine blade. Protective coating systems can include several layers including a metallic bond or barrier coating of MCrAlY having an alumina scale and, for example, a columnar yttria stabilized zirconia thermal barrier, as taught in U.S. Pat. No. 4,916,022 (Solfest et al.), which can be further coated by an erosion resistant layer of alumina or silicon carbide, as taught by U.S. Pat. No. 5,683,825 (Bruce et al.).
In U.S. Pat. No. 5,780,146 (Mason et al.), 30 wt. % to 50 wt. % (50 vol. % to 60 vol. %) of hollow alumino silicate or alumina spheres of 400 micrometer to 1800 micrometer diameter, and having a high temperature capability of approximately 1300° C., were used in an aluminum phosphate matrix, for an abradable seal. The seal is used over a ceramic matrix composite shroud segment, which may comprise silicon carbide fibers in an alumina matrix. However, this invention is limited in thermal stability due to uncontrolled sphere distribution and contact, therefore, the matrix controls the thermal stability of system and limits the temperature of the system to less than 1200° C.
Fillers have also been used by Naik et al., in U.S. Pat. No. 5,064,727. There, abradable stationary seal walls, for jet turbine housings which seal opposing, rotating rotor blade tips, have a ceramic core containing from 30 vol. % to 98 vol. % solid ceramic filler, where the ceramic fills a honeycomb wall structure. This is then covered with erosion and corrosion resistant outer layer, which is made porous by uniformly dispersed, finely divided filler. The pores can be filled with ceramic, metal oxide or carbide materials. Fillers mentioned include hollow ZrO2.8YO3 ceramic spheres and solid Al2O3,SiC,TiC and BN spheres.
Other abradable honeycomb structures for use in turbines are taught in U.S. Pat. No. 4,867,639 (Strangman). There, low melting fluorides, such as BaF2, are incorporated into a stabilized zirconia or alumina matrix which, in turn, is used to fill a honeycomb shroud lining made of, for example, a metal alloy. The filling becomes molten when the rotating blade tips rub the shroud, and upon resolidification, improve the smoothness of the abraded surface. Ainsworth et al., in U.S. Pat. No. 4,639,388, teaches another variation of reinforced ceramic layers, including a honeycomb matrix for use in a turbine as abradable seals.
In U.S. patent application Ser. No. 09/261,721, filed on Mar. 3, 1999, now U.S. Pat. No. 6,235,370, a honeycomb structure having open cells was filled, and optionally overlaid, with a material containing hollow ceramic particles embedded in an interconnected ceramic matrix, to provide a composite thermal barrier composite coating having superior erosion resistance and abrasion properties for use on combustion turbine components. The hollow particles were preferably spherical and made of zirconia, alumina, mullite, ceria, YAG or the like, having an average particle size of about 200 micrometers (0.2 mm) to 1500 micrometers (1.5 mm). The steady state erosion rate, grams lost/kg erosive impacting media, of this filler was 3.2 g/kg vs. 4.6 to 8.6 g/kg for conventional TBCs. Here, the ceramic matrix comprised an interconnected open cell honeycomb structure, binding the hollow spheres together where the hollow spheres were bonded by a network of aluminum phosphate bridging bonds.
In U.S. patent application Ser. No. 09/536,742, filed on Mar. 28, 2000, a vacuum packing/impregnation method of bonding hollow geometric shapes was described, to provide abradable, thermally stable seals and the like. Both U.S. patent application Ser. Nos. 09/049,369, filed on Mar. 27, 1998, now U.S. Pat. No. 6.197,424) and Ser. No. 09/049,328 filed on Mar. 27, 1998, now U.S. Pat. No. 6,013,592), teach ceramic insulating coatings with improved erosion resistance and macroscopic closed porosity, utilizing hollow oxide-based spheres which can contact at least 3 or 4 other hollow spheres to provide improved dimensional stability at temperatures up to about 1600° C. Erosion rate, grams lost/kg erosive impacting media was 4.5 g/kg. and 7.5 g/kg.
However, none of these coatings or seal structures have optimized abradability with erosion resistance and insulating capability, minimized shrinkability and thermal mismatch, provided constrained stabilized uniform spherical porosity and adequate flexibility, and optimized thermal stability for operation substantially up to 1600° C.; all of which characteristics will be required of the next generation high temperature turbine TBCs, seals and the like, as well as in non-turbine coating applications. What is needed are high temperature coatings, and composites that fill these and other future requirements.
Also, thermally sprayed structures having hollow spheres co-sprayed to introduce porosity for either abradability or reduced thermal conductivity, are limited to small sphere sizes, typically less than 200 microns, for spraying capability. These small spheres tend to melt in plasma and result in non-spherical pores which are not thermally stable. Such small scale porosity leads to poor erosion resistance. Additionally, thermally sprayed coatings/structures for abradable seals based on co-spray of fugitive particles, for example, polyester resin particles, which are subsequently burned out to leave increased porosity, results in small, non-spherical porosity and matrix-dominated properties which limit thermal stability. The present invention has been developed in view of the foregoing, and to address other deficiencies of the prior art.
Therefore, it is one of the main objects of this invention to provide a high temperature, erosion resistant coating and material which is bondable, generally non-shrinking, abradable, flexible, thermally stable up to at least 1600° C., and which has constrained stabilized porosity and insulating properties, as well as controlled thermal conductivity and thermal expansion properties.